For two-way, real-time audio communications, referred to here generically as voice or video telephony, a user can wear a headset that includes a single earphone (also referred to as a headphone or a speaker) and a microphone, or a pair of stereo earphones and a microphone, that are connected to a host communications device such as a smart phone. The headset, which integrates the earphones with a microphone, may be connected to the host device through a 4-conductor electrical interface typically referred to as a headset plug and jack matching pair. The four conductors are used as follows: two of them are used for the left and right earphone signals, respectively; one of them connects a microphone signal; and the last one is a reference or power return, conventionally taken as the audio circuit reference potential. The plug that is at the end of the headset cable fits into a mating 4-conductor jack that is integrated in the housing of the host device. Connections are made within the host device from the contacts of the headset jack to various audio processing electronic components of the host device.
Packaging restrictions in host devices such as a smart phone or a cellular phone create difficult challenges for routing the signal and power lines. For example, the headset jack is often located distant from the main logic board on which the audio processing components are situated, so that the headset signal needs to be routed through a flexible circuit and one or more board-to-board connectors. The multiple connections increase the impedance of the connection, as well as the manner in which the connections are made namely through narrow or thin metal circuit board traces, can lead to the coupling of audio band noise during operation of the host device. In addition, with the shared nature of the headset's reference or ground contact (shared by the microphone and the earphones of the headset), further noise is produced at the output of the microphone preamplifier. The preamplifier provides an initial boost to the relatively small microphone signal that is received from the headset. The practical effect of such audio noise at the output of the microphone preamplifier is often that the listener at the far end of a telephone conversation hears an echo of her own voice, with a concomitant reduction in the quality of the sound.
Attempts to reduce (or, as generically referred to here, “cancel”) the noise at the output of the microphone preamplifier have been made. In one case, the concept of differentially sensing the microphone signal is used. For this purpose, a differential amplifier (in contrast with a single-ended amplifier) is used to only amplify the difference between the voltage at a sense point for the headset ground contact and the voltage at a sense point for the microphone signal contact. Using such a configuration, any audio voltage that may appear as noise between a local ground (local to the microphone preamplifier) and the ground that is near the headset jack or socket are largely rejected (that is, not significantly amplified), while the audio signal on the microphone signal contact is amplified.